The present invention relates to a speed control circuit for an optical scanning system driving motor for an image forming apparatus.
As an example of the image forming apparatus, a plurality of motors for driving a photoreceptor drum, an optical scanning system, a fixing roller, and the like are used in an electrophotographic copier. Although some motors among the foregoing motors are always rotated at a constant speed, the speed of the motor for driving the optical scanning system is controlled as follows: the motor speed for a moving back operation, in which the optical system returns to a reference position after an exposure scanning operation, is higher than that for a moving forward operation in which a document is scanned for exposing.
As a motor speed control method, the following is widely known: the number of rotation of the motor is detected by an encoder; pulse signals outputted from the encoder are compared with those corresponding to a target rotational speed; PID (proportional, integral, and differential) control is conducted according to the number of pulses; and the speed control is conducted when voltage-impression upon the motor is changed by the PWM (pulse width modulation) method according to the result of the foregoing comparison. Since the control is conducted according to the pulse signal in the PID control using the encoder, the control is more exactly conducted when the encoder is used which generated a larger number of pulses while a motor is rotated once. However, since processing time necessary for one PID control is approximately determined, even when pulses are generated from the encoder in a time interval which is shorter than the foregoing processing time, the PID control can not be conducted on each pulse signal.
In Japanese Patent Publication Open to Public Inspection No. 63-69476 (69476/1988), the following copier is disclosed: an encoder which generates a large number of pulses during one rotation is used so that exact speed control can be conducted even when a motor is rotated at a low speed, for example, when an enlarging copy operation is conducted; pulse signals outputted from an encoder are divided by a predetermined number of frequencies when the motor is rotated at a high speed, for example, when a reducing copy operation is conducted; and the speed control is precisely conducted when a time interval between pulses after frequency-dividing operations, is larger than the processing time of a CPU which is necessary for one speed control operation. Accordingly, the number of frequency-dividing operations of pulse signals outputted from the encoder is separately set for each magnification ratio of copy.
However, in the invention disclosed in Japanese Patent Publication Open to Public Inspection No. 63-69476 (69476/1988), since the number of frequency dividing operations of pulse signals outputted from the encoder is set for each magnification ratio of copy with respect to a predetermined rotational speed of a motor, when the rotational speed of the motor is low while the motor speed is increased or decreased, the number of frequencies of the pulse signal outputted from the encoder is small. Accordingly, the number of PID control is small, so that the speed control can not be precisely conducted, which is a problem.
Due to the foregoing, inventors of the present invention have researched a speed control circuit of a motor as follows: an encoder is used which can generate any number of pulses, enough to conduct precise speed control, while the motor is rotated once, even at a low speed while the speed of the motor is increased or decreased; pulse signals outputted from the encoder while the speed of the motor is increased or decreased are frequency-divided by a predetermined frequency dividing number; and the speed control can be more precisely conducted when an interval between pulses after frequency-dividing operations, is longer than the processing time of a CPU which is necessary for one speed control operation.
In this connection, as a method for finding the rotational speed of the motor from the pulse signal, the following method is widely known: for example, a clock pulse generator of 10 MHz is prepared; and the number of clock pulses generated during one cycle of the pulse signal outputted from the encoder is counted. When the motor speed control is conducted by using this method, interruption of processing is generated at the time of rising of the pulse signal outputted from the encoder (hereinafter, called encoder interruption processing). Then, a calculation of the PID control is conducted according to the counted number of clock pulses and a predetermined number of a target rotational speed of the motor, and PWM signals according to the result of the calculation are outputted to the motor, in this encoder processing. Further, in order to ensure that the motor speed is reached to the predetermined rotational speed after the motor has started the rotation and a predetermined time has passed, for example, interruption of processing (hereinafter, called timer interruption processing) is generated at each 20 msec; a target rotational speed of the motor, which corresponds to an elapsed time from the start of the rotation of the motor, is set in this timer interruption processing; and the target rotational speed is used in the calculation of the PID control for the encoder interruption processing.
However, when the rotational speed of the motor is low, for example, when the motor begins to start the rotation, since the number of interruption of the encoder interruption processing is small, the number of motor speed control operations is small, so that the motor speed control can not be accurately conducted, which is a problem.
In view of the foregoing, the present invention can accurately control the motor speed even when the rotational speed of the motor is low, and the frequency of the pulse signal outputted from the encoder is low.